1. Field of the Invention
The present invention provides methods of manufacturing paper and paperboard materials having increased dry and temporary wet strength, and more particularly provides a method of making paper and paperboard materials possessing increased temporary wet and dry strength, wherein the strength improving compositions do not have shelf-life and gelling problems due to premature crosslinking. The methods of the invention comprise the addition, at the paper or paperboard mill site, of a crosslinker composition comprising at least one aldehyde generating or other suitable crosslinking compound, preferably a glyoxal releasing compound, or more preferably glyoxal itself, to a 10%-50% solution of a cationic polyacrylamide to be reacted immediately prior to its addition to the fiber composition at the wet end of the paper making process. The aldehyde generating or other suitable crosslinking compound, preferably the glyoxal releasing compound, or more preferably the glyoxal itself, is combined with a cationic polyacrylamide compound and reacted for a certain time at a certain temperature to reach a desired degree of crosslinking (prior to the necessary dilution to provide uniform distribution of the reacted material in the fiber slurry) before adding it to the fiber slurry at the wet end of the paper making process.
Since these type of crosslinking reactions depend to a high degree on a good number of parameters such as time, temperature, pH, reactant concentrations and ratios; satisfactory control of the desired degree of crosslinking is a very complex task. To carry out this on-site reaction in a practical way, under precisely controlled conditions, a suitable reactor technology must be selected that is capable of accomplishing very rapid mixing and instant heating without the use of conventional heat transfer methods.
One currently known such technology is inline mixing combined with microwave heating. Another, more preferred technology applies cavitation energy for extremely rapid simultaneous mixing and heating in one step. An eminently suitable device/reactor to accomplish this task is described by J. L. Griggs in U.S. Pat. No. 5,188,090.
In certain other methods of the invention the aldehyde generating or other suitable crosslinking compound, more preferably the glyoxal releasing compound, or simply the glyoxal itself, is contacted as a spray with the drained paper or paperboard web formed from a mixture comprising a fiber slurry and a cationic polyacrylamide composition.
2. Background
A great variety of wet end additives are available for improving paper strength. These additives must have a given cationic charge to provide their molecules with sufficient affinity to be retained on negatively charged cellulose fibers.
In addition, these chemistries are commonly modified to be more effective in improving temporary wet strength by incorporating thermosetting properties through the use of crosslinking agents like glyoxal.
However, through the use of crossliners a problem arises regarding the stability and storage life of these preparations. In most cases significant dilution to as low as 8.0% active solids concentration, pH adjustment to 3.0-4.0, and lower than room temperatures are needed to ensure somewhat practical lengths of shelf lives.
Some of these currently used commercial strength additives have less than 3 weeks of storage life, especially during the summer months.
The crosslinking of starch with multi-functional reagents, which are reactive with starch hydroxyl groups, is well known. Glyoxal and polyaldehyde compounds and resins have been previously utilized as crosslinkers. Simple mixing of glyoxal with a starch dispersion rapidly affords a gel. However, glyoxal is infinitely soluble in water and does not interact efficiently with other chemicals or compositions, particularly heterogeneous materials dispersed in small quantities in large volumes of water, e.g., such as gelatinized starch molecules or cellulosic fibers present in the wet-end of the paper making process. Thus, addition of glyoxal or other low molecular weight crosslinkers directly to the wet-end of the papermaking process has not been found to provide benefit to end product of the paper making process.
U.S. Pat. No. 6,303,000 issued to Floyd et al. (Floyd '000) discloses gelatinized starch compositions crosslinked with a glyoxal resin and the use of same in paper making. The crosslinked starch composition of Floyd '000 comprise the reaction product formed by heating starch with a blocked glyoxal resin such as those resins recited in U.S. Pat. No. 4,695,606 (Floyd, '606) during the gelatinization process. The heating process forms a gelatinized starch that is crosslinked by the glyoxal resin. More particularly, Floyd '000 discloses the addition of a crosslinked gelatinized starch composition to the wet end of the paper making process. In other words, prior to addition to the wet end, the starch is heated with the blocked glyoxal resin to gelatinize the starch and induce a crosslinking reaction between the glyoxal and the starch. The Floyd '000 patent further discloses that the glyoxal resin can be pre-mixed with the starch prior to the gelatinization heating step or added during the starch gelatinization process. Floyd suggests that pre-mixing the starch and blocked glyoxal resin prior to the gelation process or addition of the blocked glyoxal resin during the gelatinization process, affords superior material having improved shelf stability.
The Floyd '606 patent describes paper binder compositions comprising a mixture of an acrylic or vinyl polymer with a blocked glyoxal resins, e.g., such as the reaction product of glyoxal and a urea or a cyclic urea. More particularly, the blocked glyoxal resin is a condensation polymer of glyoxal blocked with urea, cyclic ureas such as ethylene urea, 4,5-dihydroxyethylene urea and propylene urea, carbamates, glycols, or polyols.
In Floyd '000 the addition levels of the gelatinized starch composition demonstrated to affect a significant improvement in paper or paperboard strength are relatively high at the level of 40 lb or more dry starch composition per ton of dry pulp. It is well known in the art of papermaking that significant issues may occur when relatively high levels of starch are used to produce paper, including high cost, high levels of effluent Biological Oxygen Demand (BOD), reduction in paper opacity, machine deposits, and problems with dewatering and drying the paper or paperboard leading to reduced production rates. It would thus be desirable to have paper strength compositions that are effective at lower levels of usage.
A variety of polymeric stabilizing agents have been recited which are capable of stabilizing at lest one aldehyde residue of a plurality of glyoxal compounds. More particularly a variety of polyacrylamide or copolymers of acrylamide and an unsaturated aliphatic carboxylic acid, which have a plurality of glyoxal equivalents attached to the polymer chain through pendant amide groups of the acrylamide residues.
U.S. Pat. No. 3,556,932 teaches poly(acrylamide) substituted with glyoxal, e.g., a polymer chain with —C(O)NHCH(OH)CHO side chains. However, because of stability issues, this thermosetting polymer must be in the form of an 8.0% solution and has a shelf life of only about 24 days.
U.S. Pat. No. 5,543,446 teaches terpolymers composed of (meth)acrylamide mononomers, unsaturated aliphatic carboxylic acid monomers, and a di- or polyvinyl monomer. The terpolymers can be used to increase the wet strength of a paper web during the paper making process.
International patent publication, WO 00/11046 teaches a copolymer of acrylamide and an α,β-unsaturated carboxylic acid which has been modified with a dialdehyde such as glyoxal.
U.S. Pat. No. 7,034,087 teaches the use of aldehyde scavengers such as choline for improved stability.
U.S. Patent Application 2005/0187356 teaches the carrying out of the crosslinking reaction in two stages, in addition to using a scavenger.
As an alternative approach, it would be desirable to have a strength improving composition comprised of the reaction product of a stabilized dialdehyde generating compound, or a stabilized glyoxal compound, or only glyoxal, and a cationic polyacrylamide in the form of a solution of much greater than 8.0% solids content, available for immediate use without having to be concerned about the limited shelf-life of the said strength additive. It would also be desirable to provide methods of making paper and paperboard with increased strength using such crosslinking compositions.